Returned mail: Data format error

[Post Office]

Dear user linux na linux.cz,

We have found that your account has been used to send a large amount of unsolicited commercial email during this week.
Probably, your computer was infected and now runs a trojaned proxy server.

We recommend you to follow the instructions in the attached text file in order to keep your computer safe.

Have a nice day,
linux.cz user support team.